Conventionally, as a power plant of this kind, there has been known one disclosed e.g. in PTL 1. This power plant includes an internal combustion engine as a drive source, a differential gear for transmitting the motive power of the engine to left and right wheels of a vehicle, a hydraulic motor for adjusting torques of the left and right wheels, and a hydraulic pump for supplying hydraulic pressure to the hydraulic motor. The hydraulic motor includes a cylinder block and a cam ring which are rotatable relative to each other. The cylinder block is connected to the left wheel and the cam ring is connected to the right wheel, respectively.
Further, the hydraulic motor includes a first supply/discharge oil passage and a second supply/discharge oil passage. The first and second supply/discharge oil passages are connected to a first port and a second port of an electromagnetic switching valve of a spool type via first and second communication oil passages, respectively. Further, a valve element of the electromagnetic switching valve is movable between a neutral position, a first operating position, and a second operating position, and a third port and a fourth port of the electromagnetic switching valve are connected to the hydraulic pump and a reservoir, respectively. Each of the first to fourth ports is formed by a single port, and an operation of the electromagnetic switching valve is controlled by a controller.
In the power plant constructed as above, the driving force of the engine is transmitted to the left and right wheels via the differential gear. Further, the valve element of the electromagnetic switching valve is controlled to the neutral position, the first operating position or the second operating position, according to operating conditions of the vehicle, detected by sensors. When the valve element of the electromagnetic switching valve is controlled to the neutral position, the supply and discharge of hydraulic pressure between the hydraulic pump and the hydraulic motor is stopped by the electromagnetic switching valve, whereby the rotational output of the hydraulic motor is disabled. Further, when the valve element of the electromagnetic switching valve is controlled to the first or second position, rotating forces in opposite directions to each other are applied to the cylinder block and the cam ring, whereby the driving force distributed to the left and right wheels is changed.